Example of Zone Authority in DNS Queries
As an example of the DNS resolving process, consider the role of a recursive DNS resolver attempting to look up the address "en.wikipedia.org.". It begins with a list of addresses for the most authoritative name servers it knows about – the root zone name servers (indicated by the full stop or period), which contains name server information for all top-level domains of the Internet.
When querying one of the root name servers, it is possible that the root zone will not directly contain a record for "en.wikipedia.org.", in which case it will provide a referral to the authoritative name servers for the "org." top level domain (TLD). The resolver is issued a referral to the authoritative name servers for the "org." zone, which it will contact for more specific information. Again when querying one of the "org." name servers, the resolver may be issued with another referral to the "wikipedia.org." zone, whereupon it will again query for "en.wikipedia.org.". Since (as of July 2010) "en.wikipedia.org." is a CNAME to "text.." (which is in turn a CNAME to "text.esams.."), and the "wikipedia.org." name servers also happen to contain authoritative data for the "." zone, the resolution of this particular query occurs entirely within the queried name server, and the resolver will receive the address record it requires with no further referrals.
If the last name server queried did not contain authoritative data for the target of the CNAME, it would have issued the resolver with yet another referral, this time to the zone text... However, since the resolver had previously determined the authoritative name servers for the zone org., it does not need to begin the resolution process from scratch but instead start at zone org., thus avoiding another query to the root name servers.
There is no requirement that resolving should involve any referrals at all. Looking up en.wikipedia.org. on the root name servers always results in referrals, but if an alternative DNS root is used which is set up to contain a record for en.wikipedia.org., then the record is returned on the first query.
Read more about this topic: DNS Zone
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